Substrate inspection systems are extensively employed in the integrated circuit fabrication industry to detect defects in substrates. These systems operate by directing a light, such as a laser light, onto the substrate, sensing properties of the reflected or scattered light, converting the sensed properties into electrical signals, converting those signals to information in a memory, and then comparing the gathered information to some kind of reference information, be it historical or otherwise, to determine various properties of the measured substrate. The reference information can come from other measurements of the substrate or from some other source of information.
As the term is used herein, “substrate” includes reticles and masks that are used to pattern integrated circuits, and the wafers—either semiconducting or insulating—upon which the integrated circuits are fabricated. “Integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
To detect the relatively small defects that are found in the commensurately small geometries of modern integrated circuits, the resolution of the inspection system needs to be commensurately high. This is typically accomplished by providing a light having a wavelength that is small enough to observe these small defects. Q-switched or excimer lasers are available at wavelengths such as 213 nanometers, 193 nanometers, and 157 nanometers, which are sufficiently small as to be able to detect the small defects within a modern integrated circuit. However, currently inspection system architectures that employ low repletion rate optical sources require high optical powers in a single pulse. For a high throughput inspection system with a high sensitivity, the required power level from a q-switched or excimer laser can be high enough to damage the substrate that is being inspected. Further, these high peak powers can be sufficient to damage the optics used to illuminate the substrate and collect the reflected and scattered light.
In addition, q-switched and excimer lasers, flashlamps, and pulsed plasma extreme ultraviolet light sources tend to exhibit large pulse-to-pulse variations in power. This variation can be difficult to compensate for in the comparison process, and tends to reduce the sensitivity of the detection process. Such lasers also tend to exhibit speckle in the illumination, and this variation of in the illumination across the surface of the substrate also reduces the detection sensitivity.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.